1. Technical Field
This invention relates generally to methods for producing organic nanocrystals on substrates and more particularly to methods for producing organic nanocrystals using a combination of hydrophobic and hydrophilic self-assembling monolayers.
2. Prior Art
Crystallization from solution is an important separation and purification process in the chemical process industries. It is a primary method for the production of a wide variety of materials ranging from inorganic compounds to high value-added materials. In addition to product purity, crystallization must also produce particles of the desired size, shape, and form. Specific conditions are necessary to crystallize chemical materials with a specific polymorph crystal form and size. Such conditions include the pH, temperature, ionic strength, and specific concentrations of salts, organic additives, detergents, and impurities in the solution. As different conditions may allow the generation of different forms and sizes of the crystals, it is often difficult to achieve or obtain the specific conditions for crystallizing a crystal with a specific form and size. As such, it is often useful to screen and test conditions that may potentially be suitable for crystal growth.
Organic monolayer films have been used as an interface across which geometric matching and interactions, such as van der Waals forces and hydrogen bonding, can transfer order and symmetry from the monolayer surface to a growing crystal. Nucleation and growth of organic crystals, nucleation rates, polymorphic selectivity, patterning of crystal, crystal morphology, and orientation (with respect to the surface) can undergo modification through site-directed nucleation. This can be achieved using supramolecular assemblies of organic molecules, such as chemically and spatially specific surfaces. Compressed at the plane of water/air interface, Langmuir monolayers are mobilized by, and commensurate with, the adsorption of aggregates during crystallization.
Self-assembled monolayers (SAMs) are single layers of ordered molecules adsorbed on a substrate due to bonding between the surface and the molecular head group. SAMs are molecular units that are spontaneously formed upon certain substrates, such as gold and silicon, when immersed in an organic solvent. One of the better known methods to form SAMs is when alkanethiol molecules chemisorb on gold surfaces through the thiol head group to reproducibly form densely packed, robust, often crystalline monolayer films. The surface chemical and physical properties of the monolayer films can be controlled precisely by varying the terminal chemical functionality of the alkanethiol molecule.
SAMs and mixed SAMs lack the mobility of molecules at an air-water interface and, hence, lack the ability to adjust lateral positions to match a face of a nucleating crystal. This is especially true for SAMs of rigid thiols, for which even conformational adjustment is not possible. SAMs of 4-mercaptobiphenyls are superior to those of alkanethiolates in providing stable model surfaces, as well as in the ability to engineer surface dipole moments. Coupled with the ability to engineer surface functionalities at the molecular level, SAMs and mixed SAMs of rigid thiol offer unique surfaces for nucleation and growth of inorganic and organic crystals.
Silane SAMs have been used to promote heterogeneous nucleation and growth of iron hydroxide crystals and to study the effect of surface chemistry on calcite nucleation, attachment, and growth. For example, CaCO3 has been crystallized on surfaces of alkanethiolate SAMs on gold and SAMs of functionalized alkanethiols can control the oriented growth of calcite. Also, the heterogeneous nucleation and growth of malonic acid (HOOCCH2COOH) has occurred on surfaces of alkanethiolate SAMs on gold that terminated with carboxylic acid and with methyl groups.
U.S. Pat. No. 6,645,293 to Allan S. Myerson discloses methods for the crystallization of nano-size crystals of molecular organic compounds while operating at a low supersaturation. The methods are based on controlling the domain size available during the crystallization process. In one method, microcontacted printed SAMs with local domain area sizes ranging from 25 μm2 to 2500 μm2 and fabricated SAMs generated from electron beam lithography, are employed to control the size, orientation, phase, and morphology of the crystal. In another method, a continuous micro-crystallizer having a vessel diameter of 25 microns or less is used to ensure that that the maximum size of the crystals in two dimensions is constrained by the vessel itself. Both methods allow control of supersaturation and growth conditions, as well as manageability over crystallinity and polymorphism, and each method's domain size has the potential for further reduction.
U.S. Pat. No. 7,329,319 to Allan S. Myerson discloses a method for producing crystals and for screening crystallization conditions of chemical materials on distinct metallic islands with specific functional groups, for preparing and screening the conditions necessary to promote a specific polymorph of a crystal, and a means for testing and screening the more precise conditions suitable for achieving a desired size or form of a crystal. This patent relates to a method in which a large number of crystals of a desired size and form can be produced quickly and reproducibly for use in the testing and screening of crystallization conditions, such as the production of crystals in metastable states and the screening of the crystals for different polymorphic forms.
U.S. Pat. No. 7,329,592 to Allan S. Myerson discloses a method for producing crystals and for screening crystallization conditions of chemical materials on distinct metallic islands with specific functional groups by using multi-functional substrates comprising a plurality of self-assembled monolayers having at least two different functionalities, for preparing and screening conditions necessary to promote specific polymorphs of a crystal, and a means for testing and screening the more precise conditions suitable for achieving desired sizes or forms of a crystal. This patent relates to a method for producing crystals and a process for screening crystallization conditions, such as the conditions for the production of crystals in metastable states, using a multi-functional substrate.
Although there are methods for initiating crystallization (including nucleation rates) and growing crystals (including affecting the polymorphic selectivity, patterning of crystal, crystal morphology, and orientation of the crystals), there is a need for methods for more efficiently and successfully producing organic nanocrystals. There also is a need for a method for producing organic nanocrystals of both a desired structure and a desired size by self-assembling monolayers on substrates. It is to these needs and others that the present invention is directed.